1. Field of the Invention
The present invention relates to a process and apparatus for electrolytically cleaning metal strands, such as ferrous wires. More particularly it relates to an improved electropickling process, using alternating current, capable of effecting rapid and essentially complete removal of oxide scale, rust and other undesirable substances from the surface of iron and steel wires. The process can yield extremely clean wire surfaces suitable for subsequent finishing treatments.
2. Related Art
The term ferrous strand as used herein refers to ferrous (e.g. steel) wire, strip, sheet etc. irrespective of composition, length or cross section. The process and apparatus of the invention are however especially suitable for use in the wire industry.
The process of this invention is generally applicable to the in-line cleaning of ferrous strands of variable carbon content, including low-carbon, medium-carbon and high-carbon steel wire with up to more than 1% C. The process is particularly suitable for use in wire pickling lines and can be applied with surprising and advantageous results to carbon steel wires with more than 0.30% C, and especially above 0.45-0.50% C.
In the case of steel wire manufacturing the undesirable substances found on the wire surface frequently take the form of higher oxides (such as ferric and magnetic oxides), hydro-oxides (rust) and also foreign impurities such as dirt particles, lubricant residues, organic or inorganic matter, and often also carbonaceous and graphitic substances derived from burnt lubricant.
Due to the diversity of character of these contaminants, related to the diversity of prior mill operations and the types of wire used, it is often necessary to subject the wire surface to a sequence of cleaning and pickling steps in order to ensure proper cleaning. For removing adherent scales and subsurface penetrated (oxide) particles, it is sometimes necessary to treat steel wires in strong mineral acids and to increase both pickling temperature and residence time. These prior art measures increase the cleaning cost with the need for long processing installations and high consumption of chemicals and energy.
The use of an electrical current to intensify the cleaning action of a chemical pickling bath has been proposed. However, in methods making use of strong acids and employing electrolyzing current "assists" to accelerate the pickling process, there are often problems of overpickling, including significant loss of base metal, and also hydrogen embrittlement which is most undesirable in high-strength applications of carbon steel wires.
To overcome some of the drawbacks of prior art pickling methods, a number of specific electrolytic processes have been developed in which a direct current is applied to the steel wires, mostly in combination with specific electrolyte conditions.
Known processes of this type include for example: electrolytic D.C. pickling of steel wires in hydrochloric acid solutions under carefully controlled electrolytic conditions.
bipolar pickling of high-carbon steel wires in strong sulfuric acid solutions (450-500 g/l of H.sub.2 SO.sub.4) at a temperature of below 30.degree. C. with high current density (150 A/dm.sup.2).
electrolytic D.C. pickling in aqueous salt solutions (sulfates, chlorides) of sufficient conductivity. In the case of D.C. pickling, however, there is the disadvantage that expensive rectifying equipment is needed and also there is usually requirement for a delicate balance of electrolytic processing conditions (polarity, electrolyte temperature, electrolyte composition, etc.). A further disadvantage of direct current pickling of ferrous wire in hydrochloric acid is chlorine gas formation at the current densities needed to effect adequate cleaning. Consequently, electrolytic pickling of carbon steel wire using D.C. is not generally satisfactory and such methods are not industrially important in comparison to the use of hydrochloric acid solutions in the non-electrolytic chemical pickling of wire.
As an alternative to D.C. electrolytic treatments, the use of alternating current has been proposed but prior attempts to use ordinary A.C. (mains frequency 50-60 Hertz) as a cheap source of electrical power to electrolyze steel wires in a hydrochloric acid pickling solution have been largely unsuccessful. One particularly serious problem with the use of an electrolyzing alternating current in the pickling of medium and high-carbon steel wires, e.g. containing at least about 0.30% C and more especially above 0.45% C, is the occurrence of a smut layer on the wire surface. This smut is difficult to remove and gives the wire a dark to black appearance. It is unacceptable because poor adhesion of surface coatings applied subsequent to said pickling operation may result. The real nature and cause of the smut phenomenon are not yet fully understood (overpickling, anodic action and surface oxidation, debris of carbonaceous origin, alloying elements or steel impurities may all play a part) but its occurrence imposes severe limits on the applicability of A.C. electrolyzing current in acid cleaning of steel wires, particularly medium and high-carbon steel wires. Hence, A.C. pickling of medium and high-carbon steel wire in HCl-solutions (in theory a most attractive process) has not been industrially important up to the present time.